Method of separating acidic gases from fluid mixtures



A. sHAPlRo Filed March 9, 195.5

METHOD OF'SPARATING ACIDIC GASES FROM FLUIDVMIXTURES Oct. 30, 1956 1NVEA/TOR! ABRAHAM HAPR AGENT United States Patent METHOD OF SEPARATINGACIDIC GASES FROM FLUID MIXTURES Abraham Shapiro, Pasadena, Calif.,assignor, by mesne assignments, to Socony Mobil Oil Company, Inc., acorporation of New York Application March 9, 195B, Serial No. 341,241

5 Claims. (Cl. 204-72) This invention relates to the separation ofweakly acidic, normally gaseous substances from fluid mixtures byabsorption in aqueous amine solutions.

R. R. Bottoms, in U. S. Patent 1,783,901, December 2, 1930 (reissued asNo. 18,958, September 26, 1933), disclosed a method of extractingacid-reacting gases such as H28, CO2, and SO2 from gaseous mixtures bymeans of any of certain amines having high boiling points, or by meansof a solution of such an amine. In the Bottoms process, also known asthe Girbotol process, the absorbent liquid is first brought into contactwith the gaseous mixture to dissolve the acidic substance and issubsequently regenerated by heat at a temperature of about 100 C., whichreleases the acidic substance in gaseous form. The regenerated amine isreused to treat further quantities of the gaseous mixture in a cyclicprocess.

In practice, the amine chosen has nearly always been one of theethanolamines or a mixture of them, or diaminoisopropanol, and it hasbeen preferred to use the amine in an aqueous solution of about 10% to30% in strength. Bottoms originally recommended an aqueous vsolution oftriethanolamine as the preferred agent, but

tendency of stable amine salts to accumulate in the circulatingsolution. If an acid is too strong or too involatile to be driven off inthe regeneration step, it remains in the solution and binds up anequivalent amount of the amine, making the latter ineffective forabsorption of the weakly acidic gas. Heretofore when such an acid hasaccumulated in the solution, it has been necessary to replace the costlyamine; this has imposed a severe economic burden.

The accumulation of strong or nonvolatile acids comes about in variousways, but principally by oxidation reactions. When HCN and H28 occurtogether in the amine solution, they react in the presence of anoxidizing agent to produce the thiocyanate of the amine. The thiosulfateand, to a lesser extent, the sulfate and salts of organic acids are alsoformed by oxidation reactions. tunately, it has not been found feasibleto completely exclude-oxidizing agents from an amine absorption system.

It has been attempted to recover the amine from the stable compounds bymixing with lime or caustic soda and distilling off the free amine. Thishas not been successful except in the case of certain amines havingrather low boiling points because most of the water distills off beforethe amine volatilizes, leaving a solution Unfore' ICG of high viscosityand high salt content, with such poor heat transfer properties that thedistillation of the amine is accompanied by decomposition.

I have found that, by modifying the amine absorption process to includea partial electrolytic purification of the amine solution being returnedfrom the regeneration step to the absorption step, and by maintainingsome of the amine, not less than about one half of one percent by weightwith respect to the entire solution, in combined form throughout theprocess, it is possible to prevent accumulation of strong andnonvolatile acids at low cost, of the order of one tenth the cost ofmaintaining the activity of the solution by addition of fresh amine.

The electrolytic cells employed are of the type in which a permeablepartition is interposed between the anode and the cathode, and they areoperated in such manner as to minimize the ow of liquid (as distinctfrom the flow of ions) through the partitions in either direction.

The improved process is described in the following and is illustrated bythe accompanying drawings, in which.

Fig. l is a ow diagram illustrating the entire process;

Fig. 2 is a diagram in plan view illustrating the electrolyticpurication step; and

Fig. 3 is a cross-sectional view of the electrolytic cells.

Referring to Fig. 1, 11 is an absorption column provided internally withconventional means for bringing immiscible lluids into contact, such asbubble plates, ceramic packing shapes, or (if a liquid is to be treated)a series of agitation chambers. The fluid to be treated is introduced tothe bottom of the column by line 12. The fluidV may be a gas or ahydrocarbon liquid, but in either case it is contaminated by weaklyacidic, normally gaseous or extremely volatile substances such as HzS,CO2, HCN, or SO2..

An aqueous solution of an amine, preferably diethanolamine, isintroduced to the top of column 11 by line 13, and the two materialsmove through the column in contact with one another and at leastgenerally in countercurrent relation. The treated fluid, substantiallyfree of acid-reacting substances, is removed from the top of the columnby line 14. The fouled amine solution is taken from the bottom of thecolumn by line 15 through heat exchangers 16 land 17 to regeneratingcolumn 18, where it is introduced near the top..

ln the regenerating column, which is also provided with bubble plates orthe like, the amine solution is brought into contact with steam risingfrom the bottom of the column. This raises the temperature of thesolution to its boiling point, which is suflicient to decompose theunstable amine salts, and the gaseous products of the decomposition arecarried off bythe steam through overhead line 19 to condenser 20. In thelatter, the steam is condensed into Water which is returned to the topof the column by line 21. The uncondensed gas, which consists of waterVapor and the weakly acidic, normally gaseous material extracted fromthe uid introduced by line 12, is withdrawn from the system by line 22.

From the bottom of the column some of the solution is withdrawn andcirculated through reboiler 23, which supplies the heat necessary forgenerating the steam which passes upwardly through the column. Theregenerated amine solution, which is to be reused in absorption column11, is taken from the bottom of column 18 by line 24.

' The portion of the process described to this point is essentially thesame as in the method originally disclosed by Bottoms .andemployedcommercially in many plants. Those skilled in the .art willunderstand that numerous details, not explicitlyshown or described, areimplicit in the description.

The amine solution withdrawn from the column 18 by line 24 is fairlyfreeof weakly acid-reacting, normally gaseous or highly volatilesubstances such as HzS, CO2, and HCN. The small proportion of thosesubstances which remains in the solution creates no problem because theydo not tend to accumulate in the system. But the solution also includesstable amine salts such as the thiocyanate, the thiosulfate, and thesulfate. Since the latter salts are not decomposed in the regeneratingcolumn, they have in the past accumulated in circulating aminesolutions, thus continually reducing the amount of free amine availablefor absorbing acidic materials from the liuid being treated.

The sulte also may accumulate in the amine solution. It is true, astaught by Bottoms, that SO2 is one of the acid-reacting gases which canbe driven from an amine solution by boiling the solution, but it passesoff more slowly than the less strongly acidic gases. Therefore, in aprocess primarily intended to separate H25, for example, from a fluidmixture and in which the regenerating column is designed and operatedwith a view to optimum removal of H25 from the fouled amine solution,such SO2 as may be incidentally present in the fluid mixture or may becreated by oxidation within the system' would tend to remain in thesolution and to accumulate.

In the present process, the tendency of more or less stable amine saltsto accumulate in the amine solution is counteracted by subjecting theregenerated solution to an electrolytic purification which removes theacidic components of those salts at a rate equal to the rate at whichthe acidic components are introduced to the system and created therein.

The amine solution from line 24 is passed through heat exchanger 17,where it gives up part of its heat to the foul solution in line 15. Itleaves the heat exchanger by line 25 at a temperature favorable forelectrolysis, which may be in the range 100 F. to 130 F. Line 25 dividesinto the two branches 26 and 27, of which the former leads to theelectrolytic purilication equipment generally indicated at 28. Thisequipment is more fully illustrated in Figs. 2 and 3 and is describedbelow.

The electrolytic system receives added water by line 29, it gives offwaste gases by lines 30 and 31, and it yields an aqueous acid solutionby line 32. The electrolyzed amine solution ows from the electrolyticsystem by line 33 and joins the remainder of the lean solution owing inby-pass line 27; the combined stream then passes through heat exchanger16 wherein as much of its heat as feasible is transferred to the foulsolution in line 15.

Then the regenerated and partially purilied amine solution passes byline 34 to reserve tank 35, which serves to stabilize the volume ofsolution in the system and also to effect additional cooling. The liquidlevel in tank 35 is maintained by intermittent or continuous addition ofwater by line 36 to make up for the water lost from the system byevaporation and otherwise. For this purpose it is preferable to usewater which is free of sodium and similar strong cations. When suchcations are present in the amine solution they form sulfides,carbonates, etc. which are not decomposed in the regenerating column butare decomposed in the electrolytic cells; they therefore impose anunnecessary load upon the electrolytic battery. A slight proportion ofsodium or the like is not appreciably disadvantageous, but the metalliccations should not be permitted to accumulate in the solution. Sincethere is ordinarily no way for the strongly alkaline metals to enter thesystem except with the water added by lines 29 and 36, the use of purewater is adequate for excluding undesirable cations.

From time to time small quantities of fresh aminey are added to replacethe loss of amine by leakage, evaporation, and entrainment or solutionin effluent fluids. But the requirements for fresh amine .in thisprocess are inconsequential as compared with the requirements inprevious forms of the amine absorption process.

From tank 35 the amine solution is pumped through cooler 37 and line 13into the top of absorption column 11, thus completing the cyclicprocess.

Referring now to Fig. 2, which illustrates electrolysis equipment 28,the amine solution in line 26 flows to the cathode compartments of abattery of electrolytic cells 38--38, here shown as consisting of three.A -dilute acid solution is passed to the anode compartments of the samecells by line 39. The cells are supplied with direct current byelectrical leads 40-40.

The electrolytic battery is shown as being arranged in parallel withrespect to liquid flow and in series with respect to electric current.The choice as to arrangement of cells is principally one of convenienceonly; however, it is preferable not to place more than a few cells inseries with respect to flow of the amine solution because it is moreeconomic to subject a large volume of the solution to slight puricationthan to more completely purify a small volume.

The electrolyzed amine .solution lows from the cathode compartments byway of manifold 4l to liquid-level control box 42 which is provided withweir 43 or other means for stabilizing the liquid level in the cathodecompartments. The solution then enters line 33 and is handled as shownin Fig. l and described above.

The acidsolution from the anode compartments llows through manifold 44to box 45, provided with liquid-level control means such as Weir 46.Weirs 43 and 46 are adjusted to minimize llow of uncharged moleculesthrough the permeable partitions of the cells, in a manner which isdescribed below.

Since the acid solution increases in specific gravity during its passagethrough the anode compartment of a cell, it is preferable to introduceit to the cells near the top and to withdraw it from near the bottom.The change in specific gravity of the catholyte is ordinarilynegligible, therefore no similar arrangement o-f the inlets and outletsof the cathode compartments is provided.

The acid solution is then taken by line 47 to settling tank 48, where itis freed of suspended precipitates such as sulfur and polymerizationproducts of thiocyanic acid. A stream of the acid solution is withdrawnfrom tank 48 by line 49 and, before being returned to the electrolyticbattery by line 39, is joined by a small quantity of water from line 29.Flow of the latter is adjusted to maintain the specic gravity of theacid solution fed to the electrolytic cells less than the specificgravity of the catholyte but to avoid reducing the acidity of theanolyte to a level less than that equivalent to about 0.1% H2804.

Another stream of `acid solution is taken from tank 43 by overflow,through line 32. This constitutes the liquid acid product of theoperation. From time to time the solid acidic product is gathered fromthe bottom of tank 48.

The electrolytic cells also yield gases and vapors. Since these includetoxic or otherwise noxious substances such as hydrogen cyanide,cyanogen, hydrogen sulde, sulfur dioxide, and ammonia, it is desirableto capture them rather than to permit them to escape to the atmosphere.For this purpose, the cells are provided with covers 50-50, as shown inFig. 3, and the spaces between the electrolyte levels and the covers areconnected to manifolds 51 and 52 which lead to gas withdrawal lines 30and 31 respectively. The gases from the anode compartments and thecathode compartments are withdrawn separately to avoid the formation ofexplosive mixtures of hydrogen and to avoid clogging of the lines by`ammonium salts.

Electrolytic cells 38-38 are contained in narrow rectangular boxes 53-53made, for example, of concrete covered with a bituminous composition.The cells are divided into anode and cathode compartments by permeablepartitions 54-54, which may consist of thin plates of a rathercoarse-grained, very permeable ceramic material having at leastmoderately good mechanical strength. A material having the texture andchemical composition ofI common tire brick is satisfactory. The upperportions of the partitions, down to a line just below the liquid levels,are made impermeable as by painting or impregnating with bituminousmaterial. This serves to prevent mixture of the anode and cathode gasesby diffusion through the partitions and to prevent seepage of liquidabove the lower liquid level when the anolyte and catholyte levels arenot equal.

Ions move freely through the permeable partitions, but the llow ofliquid through the partitions is very slight and is limited to thepassage of water from the anolyte into the catholyte by electro-osmosis,i. e., the tendency of hydrogen ions to associate themselves with watermolecules and to draw the latter toward the cathode. This isaccomplished by adjusting weirs 43 and 46 to maintain the anolyte andcatholyte levels nearly equal, the departure from exact equality beingchosen to create a hydrostatic pressure dilerential from the catholytetoward the anolyte which partially counteracts the pressure differentialdue to electro-osmosis. At no point on a partition shouldelectro-osmosis be overbalanced, nor should it at any point beaccompanied by a hydrostatic pressure differential in the samedirection.

The diiference in hydrostatic pressure depends on two factors, thedifference in liquid levels and the difference in specific gravities.The component of the pressure differential due to the latter factorincreases with the depth of liquid, while the differential due toelectro-osmosis is uniform. This prevents the maintenance of an equaldegree of pressure balance over an entire partition except in a casewhere the two solutions are equal in specic gravity. However, when theamine solution fed to a cell is heavier than the acid solution, thisdeparture from uniformity is partially counteracted by stratification ofthe anolyte. Therefore it is preferable that the acid solution suppliedto a cell have a specific gravity less than that of the amine solution.

It is ordinarily satisfactory to adjust the liquid levels, in View ofthe `difference in average speciiic gravities of the two solutions in acell, in such manner as to-mantain a mean hydrostatic pressuredifferential equivalent to about .05 inch of water. When cells or groupsof cells are arranged in series with respect to flow of the acidsolution, the specific gravity of the anolyte varies considerably indifferent parts of the battery; in such cases it Ais therefore desirableto provide a greater number of liquid-level control means to maintainthe preferred differences in liquid level in the various cells.

Electrodes 55--55 and 56-56 are ilat members mounted to face all ornearly all of the submerged surfaces of parttions 54-54. Anodes 55-55preferably consist of slabs of graphite. Cathodes 56-56 may be made ofany of various conductive materials resistant to alkaline corrosion,including steel. I prefer to space the electrodes about inch from thepartitions, but this distance is not critical.

I have found that, in electrolytic cells constructed essentially asdescribed, acting upon an aqueous solution containing about 15% totalfree and combined diethanolamine and a varying proportion of acidradicals consisting predominantly of the thiocyanate and the thiosulfatewith minor quantities of sulfate, sultite, sulfide, lower mercaptides,cyanide, and undetermined anions, each cell being provided with anelectric potential of about 7 volts, the electrolysis produces about .08to .ll pound per hour of free diethanolamine from previously combineddiethanolamine per square foot of effective cathode (or anode) area,provided the proportion of combined diethanolamine does not fall belowabout one half of one percent by weight with respect to the total aminesolution. The average current density is about 30 amperes per squarefoot.

The removal of thiocyanate is surprisingly complete as compared with thedegree of removal of other anions. This, together with the fact that theminute proportion of sulfidepresent in the amine solution-actuallyincreases during-the electrolysis, suggests that in addition to theelectrolytic decomposition of the amine thiocyanate a substantialportion of the thiocyanate in the cathode compartments is chemicallydecomposed, presumably by reduction reactions caused by the nascenthydrogen at the cathodes.

The diethanolamine solution mentioned is one employed to extracthydrogen sulfide from various gaseous and liquid products of a petroleumrefinery supplied with California crude oil.

In any particular installation, the rate at which the amine tends to becombined to form salts not decomposed in the regenerating column may beobserved, and accordingly an electrolytic battery having electrode areaat least sucient to decompose the stable amine salts at the same ratemay be provided.

The size of the electrolytic battery being thus determined, the amountof amine solution to be taken by line 26 and subjected to electrolysismay be decided upon. The rate of flow of the solution through thecathode compartments should be low enough to avoid turbulence whichwould tend to disturb the electrolytic classification of acids and basesbetween the cathodes and the partitions. Since the difference betweenthe acid content of the amine solution leaving the electrolytic batteryand the acid content of the amine solution elsewhere in the system, forexample in line 25, is inversely proportional to the quantity ofsolution passed through the electrolytic cells, and since it isdesirable that this difference be low, the preferred rate of flow of thecatholyte is not greatly less than the maximum. However, it ispermissible that the rate of flow be much lower, it being required onlythat the amine solution not remain in the cells long enough to approachcomplete removal of acid components, for that would prevent theelectrolytic battery from removing the acids at its calculated capacity.

The remainder, if any, of the amine solution from line 2S is taken byby-pass line 27 and reintroduced to the system downstream from theelectrolytic equipment. By-pass 27 also serves to carry the entire flowof amine solution at times when the electrolytic battery is shut downfor cleaning or reconditioning.

A very wide range of flow rates is permissible for the circulatinganolyte. The velocity should be great enough to prevent the acidsolution from increasing in concentration, during its course through theelectrolytic battery, enough to materially exceed the catholyte inspecic gravity, and it should be low enough to avoid turbulence withinthe cells.

It is possible, by the use of electrolysis, to produce a substantiallypure solution of free amine and such other nonvolatile-bases (e. g.,NaOH) as may be present. But to do so would be uneconomic because theefliciency of the process las regards free amine produced perkilowatthour decreases as the proportion of combined amine decreases.The eliciency falls off so sharply when the proportion of combined amineapproaches zero that I believe it will very seldom be desirable tomaintain less than about one percent of combined amine in the efuentfrom the electrolytic battery vand never desirable to maintain less thanone half of one percent. The only disadvantage in maintaining a muchhigher proportion, such as 6% of combined amine is the expense of theinvestment in amine which does not function in the absorption column andserves the process only by facilitating the electrolysis.

In a particular installation and at a particular time, the optimumproportion of combined yamine to be maintained in the solution may bedetermined in view of the observed relation between electrolyticefficiency and proportion of combined amine, the rate at which acidiccomponents are to be removed, the total volume of amine solution in thesystem, the cost of electric current, the

cost of amine, and the prevailing interest rate. Since all these factorsare variable, no fixed rule concerning the optimum proportion ofcombined amine can be given. When the tendency of stable amine salts toaccumulate is slight, the optimum proportion may be in the neighborhoodof one percent or, in an extreme case, as little as one half of onepercent. When the strong and nonvolatile acids Iare created andintroduced more rapidly, the preferred amount of combined `amine to bemaintained in the solution is correspondingly greater up to about tiveor six percent.

The proportion of combined 4amine in the efuent from the electrolyticbattery (and consequently the slightly higher proportion of stable aminecompounds elsewhere in the system) may best be controlled from day today by varying the voltage. If the potential required is found toregularly exceed about 7 volts in the individual cells, the capacity ofthe battery should be increased by the installation of additional cells.

Amines other than the ethanolamines and diaminoisopropanol have butrarely been employed in the Bottoms process and there is no reason tosuppose that they will be Ichosen for the present process. However, allthe amines which are capable of forming strong aqueous solutions andwhich fall within the group defined by Bottoms in the above-mentionedpatent as being useful in his method, i. e., aliphatic and cycloparainamines having boiling points not substantially less than that of water`and having no carboxyl or carbonyl groups in the molecule, are closelysimilar to one another as regards response to electrolysis and aretherefore as suitable for use in the present process as in the originalprocess. It should be noted that, by Bottoms definition, the termaliphatic amine includes cyclic substances such as benzylamine andpiperidine, in which the nitrogen is attached to or incorporated in aring structure through the medium of one or more methylene groups.

I claim as my invention:

1. The method of separating acidic substances from a predominantlywater-insoluble fluid mixture which comprises: bringing said fluidmixture into contact with an vaqueous lamine solution whereby saidsolution dissolves said acidic substances; raising the temperature ofthe acid-bearing amine solution to its boiling point to drive off weaklyacid-ic, normally gaseous components; subjecting at least part of the'amine solution from said heating step to electrolysis to furtherincrease the proportion of free amine in said solution; and employingthe thus reactivated solution in the treatment of further quantities of'said fluid mixture.

2. The method of separating acidic substances from a predominantlywater-insoluble fluid mixture which cornprises: bringing said uidmixture into contact with an aqueous amine solution whereby saidsolution dissolves said acidic substances; raising the temperature ofthe acidbearing lamine solution to its boiling point to drive off weaklyacidic, normally gaseous components; subjecting at least part of theamine solution from said heating step to electrolysis to remove strongeracids and less volatile acids at a rate substantially equal to the rateat which said acids are introduced to the solution and created therein;and employing the thus reactivated solution in the treatment of furtherquantities of said iiuid mixture.

3. The method of separating acidic substances from a predominantlywater-insoluble iiuid mixture which comprises: bringing said uid mixtureinto contact with an aqueous amine solution whereby said solutiondissolves said acidic substances; raising the temperature of theacid-bearing amine solution to its boiling point to drive off weaklyacidic, normally gaseous components; subjecting at least part of theamine solution from said heating step to electrolysis to furtherincrease the proportion of free amine in said solution; employing thethus reactivated solution in the treatment of further quantities of saidfluid mixture; and maintaining in `said solution throughout said methoda portion of the amine, from about one half of one percent to about sixpercent by Weight with respect to the solution, in the form ofheatstable amine salts.

4. The method of separating acidic substances from a predominantlywater-insoluble fluid mixture which comprises: bringing said fluidmixture into Contact with an aqueous lamine solution whereby saidsolution dissolves said acidic substances; raising the temperature ofthe acid-bearing amine solution to its boiling point to drive off weaklyacidic, normally gaseous components; passing at least part of the aminesolution from said heating step through the cathode compartments of abattery of electrolytic cells having permeable partitions betweenelectrodes; passing an 'aqueous acid solution through the anodecompartments of said cells; and employing the thus reactivated aminesolution in the treatment of further quantities of said fluid mixture.

5. The method of separating `acidic substances from a predominantlywater-insoluble iiuid mixture which comprises: bringing said uid mixtureinto contact with `an aqueous amine solution whereby said solutiondissolves said acidic substances; raising the temperature of theacid-bearing amine solution to its boiling point to drive off weaklyacidic, normally gaseous components; passing at least part of the aminesolution from said heating step through the cathode compartments of abattery of electrolytic cells having permeable partitions betweenelectrodes; passing `an aqueous acid solution through the anodecompartments of said cells; controlling the specific gravity of saidacid solution and the liquid levels in said cells to minimize ow ofuncharged molecules through said permeable partitions; and employing thethus reactivated amine solution in the treatment of further quantitiesof said fluid mixture.

References Cited in the file of this patent UNITED STATES PATENTS Re.18,958 Bottoms Sept. 26, 1933 2,363,386 Bock Nov. 21, 1944 2,363,387Bock Nov. Z1, 1944

1. THE METHOD OF SEPARATING ACIDIC SUBSTANCES FROM A PREDOMINANTLYWATER-INSOLUBLE FLUID MIXTURE WHICH COMPRISES: BRINGING SAID FLUIDMIXTURE INTO CONTACT WITH AN AQUEOUS AMINE SOLUTION WHEREBY SAIDSOLUTION DISSOLVES SAID ACIDIC SUBSTANCES; RAISING THE TEMPERATURE OFTHE ACID-BEARING AMINE SOLUTION TO ITS BOILING POINT TO DRIVE OFF WEAKLYACIDIC, NORMALLY GASEOUS COMPONENTS; SUBJECTING AT LEAST PART OF THEAMINE SOLUTION FROM SAID HEATING STEP TO ELECTROLYSIS TO FURTHERINCREASE THE PROPORTION OF FREE AMINE IN SAID SOLUTION; AND EMPLOYINGTHE THUS REACTIVATED SOLUTION IN THE TREATMENT OF FURTHER QUANTITIES OFSAID FLUID MIXTURE.